Strategy

Some mobs are powerful mages (will have tentacles) they may attack with a homing arcane missile or 3 subsequent laser hits. Run and roll as these attacks are deadly, accurate at long ranges and may bypass objects like pillars.

If in large crowds, make sure you use a weapon with large arc, this will result in killing as many as possible in one hit. The importance of this is that they are quite fast when running towards you on sight. Because of this, a hunter should keep his distance when confronting a crowd of Celestial Mob's as they can soon swarm you. This is especially true when facing the Celestial Emissary (Boss) were of which a crowd of these are found.

They are incredibly weak to thrusting attacks.

Drops

Notes & Trivia

The Hunter can send certain survivors to Iosefka's Clinic for safety. For each survivor sent there, a non-hostile Celestial Emissary appears in a room accessible from the Forbidden Woods entrance. When killed, they will drop items belonging to the survivors.

Yooo listen up and heres a story, about a blue guy who lives in a blue worldAll day and all night, everything he sees is just blue like him, inside and outsideBlue his house with a blue little window and a blue corvetteEverything is blue for him and himself and everybody around Cause he aint got nobody to listen to...

I'm surprised no one mentioned the indirect connection they have to Dark Souls 1. It's very reminiscent of the Pisaca creatures in the Duke's Archive, ironically also once humans who were experimented on that also happen to be blue jelly monsters with tentacles. Maybe Miyazaki has a thing for the Blue Man Group...or tentacles.

[SPOILER] Now the dlc came to us, we can be 100% sure of where these guys come from. They are definitely not aliens. They are results of all experiences made in the laboratory of the healing church. It's pretty easy to find out : patients in the laboratory are getting big big heads, are malnourished. And the boss "Living Failures" are basicaly celestial mobs (blue, malnourished, using arcane attacks. We can also say that in the hunter's nightmare, they are a lot of patients in the garden of light flowers. And in the waking world... they are a lot of celestial mobs. One last clue is the fact that false Iosefka make celestial mobs from humans we send to them. So, as a conclusion: Celestial mobs are results of experiences made by the healing church in order to turn them into great ones. It was a fail as they never reach that goal, but they gain arcanic powers so it shows that it's not a 100% fail.